IWAD

From DoomWiki.org

The acronym IWAD is generally interpreted as "internal WAD"[1] and refers to a WAD file which contains all of the game data for a complete game. In order to be identified as IWAD, a file must contain "IWAD" (49 57 41 44) magic in its header.

When the game's executable file is started, one of the first things it does is attempt to locate an IWAD file. This is done based on name; the executable has the names of the Doom, Heretic or Hexen IWAD files stored in an internal list. Valid names are:

The executable sets an internal "mode" dependent on the IWAD it finds; because of this, what constitutes "a complete set" in the above lists depends on the name of the IWAD. For example, different music lumps will be used depending on whether the executable is configured to play Doom or Doom II. The executable also behaves differently in other ways: for example, loading of PWADs is disabled when loading a shareware IWAD.

Because of this, it is possible to use the executable from one Doom game with the IWAD file of another, although there are some exceptions to this due to version issues. For example, the Doom II executable cannot be used with a Final Doom IWAD, as that executable was created before Final Doom was released. (The Final Doom IWAD, however, could be renamed to doom2.wad, or loaded as a PWAD.)